Can you please help me solve this cause I feel like my consumer rights are being abused, I also have proof of that late payment which appears on the statement they sent but they don't count that

So I bought couches last year, in April this year I skipped a payment. Called them to arrange for payment and even asked if they could extend my contract by a month and they said no it's not possible since I've signed a 12 months contract with them which ends in August 2021. Ok on the 3rd June I paid what was due on my account but I've received an email telling me that I'm still left with 3 months on my account and my account will be settled end September 2021. Are they allowed to bridge our original contract with my consent. And they sent me a statement recorded with 10 payments but on the email it's mentioned that I've only paid 9 months. What I do next because these people are robbing me daylight